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Photo by Bruce A. Sorrie
Photo Copyright Bruce A. Sorrie
Least Bittern
Ixobrychus exilis
NatureServe conservation status
Global (G-rank): G5
State (S-rank): SHB
- Reason: A single historical breeding occurrence of this species in Utah is known, and its historical presence at two other locations are suggestive of breeding. The site of confirmed historical breeding in Utah was destroyed through alteration of habitat, which is the main threat to the species in Utah. The species appears to have declined in Utah.
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General information
The least bittern, Ixobrychus exilis, occurs from extreme southern Canada to northeastern Argentina. It is of occasional occurrence in Utah, being an extremely rare breeder at a few scattered locations in the state during summer. It inhabits freshwater and brackish marshes with dense, tall emergent vegetation such as cattails, sedges, and bulrushes. The diet of the least bittern consists mainly of small fishes, but snakes, frogs, tadpoles, aquatic invertebrates, small mammals, and the eggs and nestlings of birds are also taken.
The nest, a platform with an overhead canopy, is constructed mostly by the male and is usually placed over water one-half foot to three feet up in emergent marsh vegetation, but rarely may be on the ground. The clutch consists of two to seven eggs, which are incubated by both parents for nineteen to twenty days. Both parents feed the young, which fledge after another twenty-five days. Two broods are commonly produced each nesting season.
Species range
This species is known from the marshes around the Great Salt Lake, the Bear River Gun Club and Bear River Migratory Bird Refuge (Box Elder County), and along the Virgin River (Washington County). It formerly bred in the area of Hot Springs Lake north of Salt Lake City (Salt Lake County), before this site was drained (Hayward et al. 1976). Gibbs et al. (1992), in their thorough account of this species in North America, apparently considered it to be so scarce in this state that they did not indicate its presence in Utah in their range map.
Migration
Arrives at nesting areas in the northeastern U.S. in early to mid-April or early May (Bent 1926); arrival may be delayed until late April in more northern states, including Maine (Palmer 1949) and New York (Swift 1987). Generally leaves northern breeding areas by September-October (Palmer 1962). Migrates apparently at night. Breeding populations south of the U.S. are mostly sedentary. North American birds migrate as far south as Colombia. Seasonal movements in Costa Rica are related to water levels (Stiles and Skutch 1989). Young may linger at nesting areas until October before beginning their southward migration (Palmer 1962).
Habitat
Hayward et al. (1976) mentioned the breeding occurrence of this species "at least formerly in the marshes around Great Salt Lake." Walters and Sorensen (1983) indicated its Utah habitat to be "marshes and wet hummocks".
Food habits
Eats small fishes, amphibians, leeches, slugs, snails, crustaceans, insects, and occasionally small mammals (Palmer 1962); possibly the eggs and young of marsh-nesting blackbirds. Feeds young by regurgitation. Forages in shallow water or along banks. Heavy growths of cattail, bulrush, wild rice, burreed, water smartweed, and reeds are favored feeding sites (Brewer et al. 1991). The repertoire of feeding behaviors is relatively small (Kushlan 1978), and among 28 recognized behaviors used by herons, only four are employed: standing in place; walking slowly; neck swaying (to overcome glare, to increase camouflage, or to have muscles in movement when strike begins); and wing-flicking, which involves quick, repeated extension and retraction of wings that may startle prey from hiding (Sutton 1936). The small size and highly compressed trunk enable it to move easily through dense vegetation. It forages by stalking along branches and reeds, and often clings to clumps of vegetation above water-level, aided by its short, outer toes and long, curved claws. It may build foraging platforms of bent reeds at productive feeding sites (Weller 1961, Reid 1989), and frequent these platforms during late-incubation and brood-rearing periods (Reid 1989). Clinging to emergent vegetation and constructing platforms enables it to forage at water depths as great as those of the largest North American herons (25-60 cm depth). The small, thin bill is probably an adaptation for securing small, fast-moving prey (Kushlan 1978), an observation supported by dietary analyses. Contents of 93 stomachs from Florida included 40% fish (top minnows, mud minnows, yellow perch, and sunfish), 21% dragonflies, 12% other aquatic insects, and 10% crustaceans (mainly crayfish (Howell 1932)). Six stomachs from Pennsylvania contained small mammals, beetles, fish, a snake, and vegetable matter (Warren 1890). Nesting birds consumed damselflies, grasshoppers, other insects, and tadpoles in Iowa (Weller 1961). Also may prey on the eggs and young of yellow-headed blackbirds (XANTHOCEPHALUS XANTHOCEPHALUS) (Roberts 1936, cited in Hancock and Kushlan 1984). Other foods include frogs, lizards, mollusks, and leeches (Bent 1926).
Ecology
Usually solitary, secretive. In six tidal marshes along the Hudson River in New York, Swift (1987) observed an average of up to 0.5 individuals per ha during playback surveys using tape-recorded vocalizations in 1986 and 1987. Other reports of nesting densities in North America vary widely: 15 nests per ha (n = 15 nests: Palmer 1962), one nest per ha (n = 26 nests: Palmer 1962), and one nest per ha (n = 19 nests: Kent 1951). Approximately equal numbers of males and females were observed by Frederick et al. (1990) during the breeding season in the Florida Everglades.
Reproductive characteristics
Nesting densities can be high (e.g. 15 nests in 0.8 ha, Wood 1951), but are often on the order of 1/hectare (Palmer 1962, Kent 1951).In the north-central U.S., nests in June-July (Brewer et al. 1991). In southern Florida, most nesting was initiated after 18 May, with large numbers of fledglings appearing after 15 June (Frederick et al. 1990). Peak nesting (including first and second broods) occurs in the northeastern U.S. from late-May to early-July (Palmer 1962, Swift et al. 1988). Copulation occurs on the nest before and during incubation (Weller 1961). Females lay one egg daily to complete clutches of two to seven eggs, usually four to five eggs (Trautman 1940, Kent 1951, Weller 1961, Hansen 1984). Clutch-size seems to increase with latitude in North America (Palmer 1962). Early clutches, laid before mid-June, may be smaller than later clutches (Weller 1961). Incubation by both sexes begins upon deposition of the first or second egg, and lasts 17-20 days (Weller 1961, Aniskowicz 1981). The semi-altricial young hatch asynchronously over a three day period (Weller 1961, Palmer 1962). Young, brooded for several days, leave the nest after five to 15 (usually 10) days but return occasionally to be fed regurgitated food by their parents, mostly by the male (Gabrielson 1914, Weller 1961, Cogswell 1977). Hansen (1984) and Nero (1950) noted that the youngest hatchling among broods grew the slowest because it received less food. Can probably rear second broods (Kent 1951, Weller 1961). Although few data are available on territoriality and mating systems, most nest solitarily and probably form seasonal, monogamous pair-bonds (Weller 1961, Palmer 1962). Occasionally breeding colonies form near abundant food sources (Audubon 1840, Weller 1961, Palmer 1962, Kushlan 1973). Descriptions of courtship and territorial behavior are given by Weller (1961). Age of first breeding is unknown (Palmer 1962). NESTING SUCCESS: Success of 38 nests was estimated by Weller (1961) to be 84% (nests in which one or no eggs hatched), with an average of three young hatched per nest. Kent (1951) observed that eggs in 70% of 20 nests hatched, and listed abandonment, predation, cannibalism, and disease as causes of nest loss. In Kansas, four of five nests studied by Hansen (1984) were depredated, at least one by a raccoon (PROCYON LOTOR). High water also may destroy nests (McVaugh 1975), marsh wrens (CISTOTHORUS PALUSTRIS) may puncture eggs (Bent 1926), and mink (MUSTELA VISON) may take eggs and nestlings (Hancock and Kushlan 1984).
Threats or limiting factors
Gibbs et al. (1992) stated: "Destruction of wetland habitat is likely the greatest threat to this species"; they also mentioned pollution, acid precipitation, siltation, pesticides, and invasive plant species among threats. They noted, too: "Because Least Bitterns fly low to the ground, collisions with motor vehicles, barbed-wire fences, and transmission lines can be a significant mortality factors . . . [and] [a]irboats are another danger; 3% of 607 birds flushed by airboats were struck . . .."
References
- Gibbs, J. P., F. A. Reid, and S. M. Melvin. 1992. Least bittern. Birds of North America 17: 1–11.
- Ehrlich, P. R., D. S. Dobkin, and D. Wheye. 1988. The birder’s handbook[:] a field guide to the natural history of North American birds. Simon & Schuster, New York. xxx + 785 pp.
